In 1760, the Appalachian Mountains served as the primary boundary to westward settlement in the British colonies.

Here's why:

* Natural barrier: The Appalachian Mountains formed a formidable natural barrier, challenging transportation and communication.

* Proclamation of 1763: This proclamation, issued by the British government, forbade settlement west of the Appalachian Divide. This was intended to appease Native American tribes and prevent further conflict.

* Limited exploration: While some explorers had ventured into the Ohio Valley and other areas west of the mountains, large-scale settlement was still limited.

While the Appalachian Mountains weren't the only factor, they were the most significant geographical feature preventing widespread westward expansion in 1760.